Embodiments of the invention are directed, in general, to communication systems and, more specifically, to enabling unicast signaling from user equipments UEs to a serving base station BS during transmission time intervals that the base station transmits multicast/broadcast signaling to user equipments UEs.
The global market for both voice and data communication services continues to grow as does users of the systems which deliver those services. As communication systems evolve, system design has become increasingly demanding in relation to equipment and performance requirements. Future generations of communication systems, will be required to provide high quality high transmission rate data services in addition to high quality voice services. Orthogonal Frequency Division Multiplexing (OFDM) is a technique that will allow for high speed voice and data communication services.
Orthogonal Frequency Division Multiplexing (OFDM) is based on the well-known technique of Frequency Division Multiplexing (FDM). OFDM technique relies on the orthogonality properties of the fast Fourier transform (FFT) and the inverse fast Fourier transform (IFFT) to eliminate interference between carriers. At the transmitter, the precise setting of the carrier frequencies is performed by the IFFT. The data is encoded into constellation points by multiple (one for each carrier) constellation encoders. The complex values of the constellation encoder outputs are the inputs to the IFFT. For wireless transmission, the outputs of the IFFT are converted to an analog waveform, up-converted to a radio frequency, amplified, and transmitted. At the receiver, the reverse process is performed. The received signal (input signal) is amplified, down converted to a band suitable for analog to digital conversion, digitized, and processed by a FFT to recover the carriers. The multiple carriers are then demodulated in multiple constellation decoders (one for each carrier), recovering the original data. Since an IFFT is used to combine the carriers at the transmitter and a corresponding FFT is used to separate the carriers at the receiver, the process has potentially zero inter-carrier interference such as when the sub-carriers are separated in frequency by an amount larger than the maximum expected Doppler shift.
FIG. 1 is a diagram illustrative of the Frequency 103—Time 101 Representation 100 of an OFDM Signal. In FDM different streams of Information are mapped onto separate parallel frequency channels 140. Each FDM channel is separated from the others by a frequency guard band to reduce interference between adjacent channels.
The OFDM technique differs from traditional FDM in the following interrelated ways:                1. multiple carriers (called sub-carriers 150) carry the Information stream;        2. the sub-carriers 150 are orthogonal to each other, and        3. a Cyclic Prefix (CP) 110 (also known as guard interval) is added to each symbol 120 to combat the channel delay spread and avoid OFDM intersymbol interference (ISI).        
The data/Information carried by each sub-carrier 150 may be user data of many forms, including text, voice, video, and the like. In addition, the data includes control data, a particular type of which is discussed below. As a result of the orthogonality, ideally each receiving element tuned to a given sub-carrier does not perceive any of the signals communicated at any other of the sub-carriers. Given this aspect, various benefits arise. For example, OFDM is able to use orthogonal sub-carriers and, as a result, thorough use is made of the overall OFDM spectrum. As another example, in many wireless systems, the same transmitted signal arrives at the receiver at different times having traveled different lengths due to reflections in the channel between the transmitter and receiver. Each different arrival of the same originally-transmitted signal is typically referred to as a multi-path. Typically, multi-paths interfere with one another, which is sometimes referred to as interSymbol interference (ISI) because each path includes transmitted data referred to as symbols. Nonetheless, the orthogonality implemented by OFDM with a CP considerably reduces or eliminates ISI and, as a result, often a less complex receiver structure, such as one without an equalizer (one-tap “equalizer” Is used), may be implemented in an OFDM system.
The Cyclic Prefix (CP) (also referred to as guard interval) is added to each symbol to combat the channel delay spread and avoid ISI. FIG. 2 is a diagram illustrative of using CP to eliminate ISI and perform frequency domain equalization. Blocks 200 each comprising cyclic prefix (CP) 210 coupled to data symbols 220 to perform frequency domain equalization. OFDM typically allows the application of simple, 1-tap, frequency domain equalization (FDE) through the use of a CP 210 at every FFT processing block 200 to suppress multi-path interference. Two blocks are shown for drawing convenience. CP 210 eliminates inter-data-block interference and multi-access interference using Frequency Division Multiple Access (FDMA).
Since orthogonality is typically guaranteed between overlapping sub-carriers and between consecutive OFDM symbols in the presence of time/frequency dispersive channels, the data symbol density in the time-frequency plane can be maximized and high data rates can be very efficiently achieved for high Signal-to-Interference and Noise Ratios (SINR).
FIG. 3 is a diagram illustrative of CP insertion. A number of samples is typically inserted between useful OFDM symbols 320 (guard interval) to combat OFDM ISI induced by channel dispersion, assist receiver synchronization, and aid spectral shaping. The guard interval 310 is typically a prefix that is inserted 350 at the beginning of the useful OFDM symbol (OFDM symbol without the CP) 320. The CP duration 315 should be sufficient to cover most of the delay-spread energy of a radio channel impulse response. It should also be as small as possible since it represents overhead and reduces OFDM efficiency. Prefix 310 is generated using a last block of samples 340 from the useful OFDM symbol 330 and is therefore a cyclic extension to the OFDM symbol (cyclic prefix).
When the channel delay spread exceeds the CP duration 315, the energy contained in the ISI should be much smaller than the useful OFDM symbol energy and therefore, the OFDM symbol duration 325 should be much larger than the channel delay spread. However, the OFDM symbol duration 325 should be smaller than the minimum channel coherence time in order to maintain the OFDM ability to combat fast temporal fading. Otherwise, the channel may not always be constant over the OFDM symbol and this may result in Inter-sub-carrier orthogonality loss in fast fading channels. Since the channel coherence time is inversely proportional to the maximum Doppler shift (time-frequency duality), this implies that the symbol duration should be much smaller than the inverse of the maximum Doppler shift.
The large number of OFDM sub-carriers makes the bandwidth of individual sub-carriers small relative to the total signal bandwidth. With an adequate number of sub-carriers, the Inter-carrier spacing is much narrower than the channel coherence bandwidth. Since the channel coherence bandwidth is inversely proportional to the channel delay spread, the sub-carrier separation is generally designed to be much smaller that the Inverse of the channel coherence time. Then, the fading on each sub-carrier appears flat in frequency and this enables 1-tap frequency equalization, use of high order modulation, and effective utilization of multiple transmitter and receiver antenna techniques such as Multiple Input/Multiple Output (MIMO). Therefore, OFDM effectively converts a frequency-selective channel into a parallel collection of frequency flat sub-channels and enables a very simple receiver. Moreover, in order to combat Doppler effects, the Inter-carrier spacing should be much larger than the maximum Doppler shift.
By assigning transmission to various simultaneously scheduled UEs in different RBs, the Node B scheduler can provide intra-cell orthogonality among the various transmitted signals. Moreover, for each individual signal, the presence of the cyclic prefix provides protection from multipath propagation and maintains in this manner the signal orthogonality.
Each scheduled UE is informed of the scheduling assignment through the downlink (DL) control channel. The scheduling assignment can be for a signal transmission from the Node B and reception by a UE (downlink scheduling assignment) or for a signal transmission from the UE and reception at the Node B (uplink scheduling assignment). The control channel typically carries the scheduled UE identities (IDs), RB assignment information, the MCS used to transmit the data, the transport block size, and hybrid ARQ (HARQ) information relating to possible data packet re-transmissions. The control channel may optionally carry additional information such as for a multi-input multi-output (MIMO) transmission scheme. A scheduling assignment may relate either to data transmission from the Node B to a UE (downlink of a communication system) or to data transmission from a UE to the Node B (uplink of a communication system).
DL communication may involve dedicated communication from one or more serving Node Bs to multiple UEs in a unicast mode. This mode implies that the transmitted data signal carries information that is specific to a single UE. DL communication may also involve multicast/broadcast communication from one or more serving Node Bs to multiple UEs in a multicast/broadcast mode. This mode implies that the transmitted data signal carries information that is intended to multiple UEs (common information data content). For example, a unicast transmission may be a file download by a UE while broadcast/multicast transmission may be the broadcasting of news.
When the available communication bandwidth is not excessively large, a preferable method to multiplex the unicast and multicast/broadcast modes of communication is through time division multiplexing (TDM). With TDM, unicast communication exists during certain transmission time intervals (TTIs) of a frame while multicast/broadcast communication may commence during the remaining TTIs.
In TTIs supporting the multicast/broadcast mode, no unicast communication exists in the DL. However, it is still possible to transmit unicast data and associated control signaling in the uplink (UL) of the communication system. Otherwise, substantial bandwidth resources can be wasted in the UL.
To enable scheduling of data transmissions in the UL, a corresponding control channel carrying the respective scheduling assignments for unicast communication needs to be transmitted during multicast/broadcast TTIs.
Thus, there is a need to maximize the spectrum use in the uplink of a communication system employing time division multiplexing of unicast and multicast/broadcast communication modes in the downlink.
There is another need to multiplex unicast control signalling for scheduling assignments of uplink data signal transmissions in TTIs supporting the multicast/broadcast communication mode.
There is another need to minimize the overhead of unicast control signaling in TTIs supporting the multicast/broadcast communication mode.